1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus (hereinbelow will be called as MRI apparatus) which reconstructs images of tissue of a living inspection body and, in particular, relates to a magnet device of the MRI apparatus which is provided with an improved magnetic field correcting means, in that a pole piece, for enhancing static magnetic field uniformity in a magnetic field region where image photographing of the living inspection body is performed.
2. Conventional Art
In some of MRI apparatuses which reconstruct images of tissue of a living inspection body, the static magnetic field is generated by making use of superconducting coils. FIGS. 1A and 1B are diagrams showing an example of conventional MRI apparatus in which superconducting coils are used. FIG. 1A is a schematic outlook of the MRI apparatus, and FIG. 1B is a cross sectional view of FIG. 1A seen from arrow A in FIG. 1A. In this MRI apparatus, static magnetic field is generated by means of a pair of superconducting coils 13A and 13B which are opposingly disposed invertical direction. The superconducting coils 13A and 13B are supported by magnetic plates 15A and 15B disposed opposingly in vertical direction. The magnetic plates 15A and 15B are mechanically supported by post shaped yokes 16A through 16D disposed therebetween. Inside the superconducting coils 13A and 13B, pole pieces 14A and 14B made of iron and for providing a desirable static magnetic field uniformity are provided, and are supported by the magnetic plates 15A and 15B together with the superconducting coils 13A and 13B. The pole pieces 14A and 14B operate to enhance the static magnetic field uniformity in a uniform static magnetic field region 11. The magnetic plates 15A and 15B and the post shaped yokes 16A through 16D serve as magnetic passages for magnetic fluxes generated by the superconducting coils 13A.
In the thus constituted MRI apparatus, a magnetic circuit is formed which is constituted by the superconducting coils 13B, the pole piece 14B, the uniform static magnetic field region 11, the pole piece 14A, the superconducting coil 13A, the magnetic plate 15A, the post shaped yokes 16A through 16D and the magnetic plate 15B, thereby, uniform static magnetic field in vertical direction is generated in the uniform static magnetic field region 11. Photographing of magnetic resonance images is performed in this uniform static magnetic field region 11. Gradient magnetic field coils 18A and 18B used in the magnetic circuit are configured in generally flat plate shapes as shown in FIG. 1B and are opposingly disposed in vertical direction while interposing the uniform static magnetic field region 11 therebetween. The gradient magnetic field coils 18A and 18B are respectively received within the recessed portions of the pole pieces 14A and 14B.
It is well known that through activation of the gradient magnetic field coils 18A and 18B, eddy current is induced on conducting portions such as the pole pieces 14A and 14B and the superconducting coils 13A and 13B, and residual magnetism is also induced on ferromagnetic bodies such as the pole pieces 14A and 14B due to their hysteresis, all of which greatly affect adversely on the images to be reconstructed. In order to prevent generation of such eddy current and residual magnetism, it has been conventionally known to dispose eddy current suppressing members 19A and 19B such as a high magnetic permeability material having eddy current limiting property on the inner face of the recessed portions of the pole pieces 14A and 14B. For example, U.S. Pat. No. 5,061,897 discloses a provision of eddy current limiting member of high magnetic permeability material on the inner surface of the recessed portions of pole pieces. Further, JP-A-9-117431 (1997) discloses a provision of a plurality of blocks formed by laminating and integrating silicon steel sheets on the inner surface of the recessed portions of the pole pieces. Still further, JP-A-1-86954 (1989) discloses to constitute the inner surface of the recessed portions of the pole pieces by a magnetic material having high magnetic permeability and a compound magnetic material in such a manner that the magnetic material having high magnetic permeability locates next to the space of the static magnetic field region.
Although, all of the above conventional art discloses the provision of a low eddy current material or a high magnetic permeability material on the inner surface of the recessed portions of the pole pieces, none of the conventional art suggests working of the inner surface of the recessed portion of the pole pieces, in order to regulate static magnetic field uniformity in the uniform static magnetic field region. This is because mechanical working of such as the high permeability material having eddy current limiting property provided on the inner surface of the recessed portions of the pole pieces is generally difficult which prevents high precision working, and if working is possible which requires high cost. For this reason, with the conventional art it was difficult to sufficiently enhance the uniformity of the static magnetic field.
An object of the present invention is to provide an MRI apparatus having a magnet device with a pole piece which enhances uniformity of the static magnetic field region thereof as well as suppresses adverse effect by eddy current and residual magnetism induced on the pole piece by the gradient magnetic field coils.
An MRI apparatus of the present invention is provided with a pair of static magnetic field generating means which are disposed in opposing manner and cause to generate static magnetic field in the space region therebetween; a pair of static magnetic field correction means which enhance uniformity of the static magnetic field in the static magnetic field region; a pair of gradient magnetic field generating means which cause to generate gradient magnetic field in the static magnetic field region; and a pair of magnetic plate means which hold the static magnetic field generating means, the static magnetic field correction means and the gradient magnetic field generating means, wherein the static magnetic field corrections means is provided with a pair of static magnetic field uniformity controlling means of which cross sectional shapes are configured so as to enhance the uniformity of static magnetic field in the static magnetic field region and which are disposed adjacent to the respective magnetic plate means and a pair of flat plate shaped suppressing means which shield adverse effect of the gradient magnetic field generated by the gradient magnetic field generating means on the static magnetic field uniformity controlling means and which are disposed adjacent to the gradient magnetic field generating means. Wherein the static magnetic field uniformity controlling means is, for example, constituted by iron which is easily worked into a desired cross sectional shape which enhances uniformity of static magnetic field in the static magnetic field region, and the suppressing means, which is for preventing adverse effect on the static magnetic field uniformity controlling means of the pole pieces caused by the gradient magnetic field generated from the gradient magnetic field generating means, is worked and manufactured easily only by cutting out a flat plate of high magnetic permeability material having eddy current limiting property such as silicon steel sheet and soft ferrite into a simple disk while following the shape of the gradient magnetic field generating means.
In other words, in the present invention, a function of suppressing eddy current and residual magnetism induced on the static magnetic field uniformity controlling means by the gradient magnetic field from the gradient magnetic field coils added thereto is separated, and the portion functioning as suppressing the eddy current and residual magnetism which requires a low eddy current and high magnetic permeability material unsuited for precision working is constituted only by cutting out a flat plate of such material into a simple disk while following the shape of the adjacent gradient magnetic field generating means, and on the other hand, the portion of the original uniform static magnetic control function which requires a complex cross sectional shape is constituted by working an easily workable material, for example, soft iron, into a desired cross sectional shape with a high precision.
Further, the MRI apparatus of the present invention is further provided with a spacer means in a space between the static magnetic field uniformity controlling means and the suppressing means. Through the provision of the spacer means the suppressing means is fixed firmly and precisely.
Still further, the MRI apparatus of the present invention is further provided with a reinforcing means between the suppressing means and the spacer means for reinforcing the suppressing means. Since the suppressing means is constituted by adhering small pieces of such as silicon steel sheets and soft ferrite in a plane configuration and in multi-layers, it is difficult to obtain a sufficient mechanical strength. Therefore, through the provision of the spacer means and the reinforcing means between the static magnetic field uniformity controlling means and the suppressing means, the mechanical strength of the suppressing means is sufficiently enhanced.